Add lintr config

[JanMarvin]

Currently there are hundreds of notifications that lintr throws. There are many cases where lintr follows the tidyverse style by default, while we use a somewhat wild mix of whatever the authors liked. Fixing many of these style-related clues is a tedious task (that no one is going to tackle anytime soon, and - on a personal note - some of the default style ideas are at least debatable), but currently there is no longer any benefit from static code analysis if the output is buried in a bunch of stuff that no one cares about. So I disabled all the non-critical linters to just show the warnings for now.

Some of the other stuff should be fixed as well, but until someone has the time to fix those, I think it's best to just not output them at all.

[JanMarvin]

God damnit. Guess now I have to fix all the broken exports/importsfrom 😞

[JanMarvin]

That's much better. The remaining warnings are actual warnings we could/should work on. Either by adding #nolint or better by fixing them.

Currently the following is reported:

Warning: local variable ‘n’ assigned but may not be used
Warning messages:
1: In readLines(filename) :
  incomplete final line found on '/Users/runner/work/openxlsx/openxlsx/tests/testthat/test-loading_workbook.R'
2: In readLines(filename) :
  incomplete final line found on '/Users/runner/work/openxlsx/openxlsx/tests/testthat/test-named_regions.R'
3: In readLines(file) :
  incomplete final line found on '/Users/runner/work/openxlsx/openxlsx/tests/testthat/test-named_regions.R'
4: In readLines(filename) :
  incomplete final line found on '/Users/runner/work/openxlsx/openxlsx/tests/testthat/test-outlines.R'
5: In readLines(filename) :
  incomplete final line found on '/Users/runner/work/openxlsx/openxlsx/tests/testthat/test-saveWorkbook.R'
6: In readLines(filename) :
  incomplete final line found on '/Users/runner/work/openxlsx/openxlsx/tests/testthat/test-write_xlsx_vector_args.R'
Warning: local variable ‘count’ assigned but may not be used
Warning: local variable ‘sheetWithPivot’ assigned but may not be used
Warning: no visible global function definition for ‘shell’
Warning: local variable ‘nvars’ assigned but may not be used
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: local variable ‘dateIds’ assigned but may not be used
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid length(...):1 expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid length(...):1 expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: local variable ‘bgFillList’ assigned but may not be used
Warning: local variable ‘fgFillList’ assigned but may not be used
Warning: local variable ‘xml’ assigned but may not be used
Warning: local variable ‘sheet’ assigned but may not be used
Warning: local variable ‘sheet’ assigned but may not be used
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:ncol(...) expressions, use seq_len.
Warning: Avoid 1:ncol(...) expressions, use seq_len.
Warning: Avoid 1:nrow(...) expressions, use seq_len.
Warning: Avoid 1:nrow(...) expressions, use seq_len.
Warning: Avoid 1:ncol(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.
Warning: Avoid 1:length(...) expressions, use seq_len.

[ycphs]

I cleaned up the code with PR #168